1. Field of the Invention
The present invention relates to a light amount control apparatus for controlling the light amount of an LED element, a control method therefor, and a display apparatus.
2. Description of the Related Art
In recent years, some display apparatuses such as a liquid crystal monitor use a light emitting diode (LED) as a light source to reduce the power consumption and to improve the contrast ratio. An LED element of a display apparatus has a light amount which can be changed by, for example, a pulsed current, and is activated by feedback control of the pulsed current so that light converges to a set given light amount in normal use. A display apparatus using an LED element as a light source requires a time for feedback control from when the power is turned on until light converges to a predetermined light amount.
Since an LED element has characteristics in which a voltage applied to the element changes according to a change in temperature due to heat of the element itself, the light amount changes in proportion to the temperature. That is, since a current necessary for achieving a target light amount is different depending on the ambient temperature of the LED element, a time required from when the power is turned on until light converges to a desired light amount may become long depending on the temperature if a pulsed current supplied to the element upon power-on is fixed. For example, Japanese Patent Laid-Open No. 2006-171693 discloses a technique of shortening a time required from when the power is turned on until light converges to a desired light amount by correcting, depending on the temperature, a preset pulsed current (initial pulsed current) which is supplied to an LED element upon power-on.
It is known that the light amount of an LED element decreases due to degradation of an encapsulation resin with aging. That is, since a correspondence between a current value and a light amount changes depending on aging similarly to a change in temperature, a time required from when the power is turned on until light converges to a predetermined light amount becomes long if a pulsed current which is supplied to the element upon power-on is fixed. For example, Japanese Patent Laid-Open No. 2006-171695 discloses a technique of shortening a time required until the light amount of a predetermined chromaticity is obtained, by storing the ratio of a current value when light actually converges to a desired light amount to the current value of an initial pulsed current, and multiplying the initial pulsed current by the ratio.
Japanese Patent Laid-Open Nos. 2006-171693 and 2006-171695 described above, however, disclose techniques of separately solving the influences of the ambient temperature and aging on light amount control of the LED element but do not disclose any technique of simultaneously solving the influences.
Japanese Patent Laid-Open No. 2006-171695 does not consider the influence of an ambient temperature when the current value ratio (aging correction coefficient) for correcting the influence of aging on light amount control of the LED element is stored. That is, the aging correction coefficient in Japanese Patent Laid-Open No. 2006-171695 contains the influence of the ambient temperature of the LED element when it is stored. If, therefore, an ambient temperature upon power-on is different from that when storing the aging correction coefficient, it may be impossible to obtain a pulsed current suitable for obtaining a desired light amount even though the initial pulsed current is multiplied by the aging correction coefficient.
Furthermore, if pulsed current correction using a thermal correction coefficient based on the ambient temperature (standard temperature) of the LED element when the initial pulsed current is set, like Japanese Patent Laid-Open No. 2006-171693, is additionally executed, it may be impossible to obtain an appropriate pulsed current. In some cases, the obtained pulsed current may be farther from an appropriate value.
Consider a case in which the ambient temperature when storing the aging correction coefficient is higher than the standard temperature according to Japanese Patent Laid-Open No. 2006-171695. In this case, since the obtained aging correction coefficient contains the influence of the ambient temperature, it may be larger than an aging correction coefficient calculated for the standard temperature. Assume that the ambient temperature upon power-on is the standard temperature. In this case, if the initial pulsed current is multiplied by the aging correction coefficient, an obtained pulsed current value may be unwantedly high. Since, however, the thermal correction coefficient in Japanese Patent Laid-Open No. 2006-171693 is 1 at the standard temperature, it may be impossible to correct the pulsed current to an appropriate value even though the pulsed current is multiplied by the thermal correction coefficient. Alternatively, if the ambient temperature upon power-on is the same as that when storing the aging correction coefficient (higher than the standard temperature), it is possible to obtain an appropriate pulsed current at this time by multiplying a default pulsed current by the aging correction coefficient. Since, however, correction according to Japanese Patent Laid-Open No. 2006-171693 is subsequently executed, a finally obtained pulsed current value may be larger than an appropriate value (because the thermal correction coefficient is larger than 1 when the ambient temperature is higher than the standard temperature).